The present invention relates to a GPS receiver, and in particular relates to a multi-channel type GPS receiver which is able to shorten the search time during a cold start.
Furthermore, the invention relates to a GPS receiver by which a flexible channel configuration is enabled.
A GPS satellite sends electric wave signals in two frequency bands called L1 and L2.
This wave signal is such that the pseudonoise code (hereinafter called PN code) and navigation message are modulated by a DS spread spectrum modulation system.
The L1 wave is modulated by PN codes containing C/A codes (Coarse Acquisition codes) with a chip rate of 1/023 Mbps and code length of 1 ms, and a P code (Precision codes) with a chip rate of 10.23 Mpbs and code length of 7 DAYS, and the L2 wave is modulated by only normal P codes.
FIG.2 shows the general construction of an analog front end GPS receiver, which demodulates L1 and L2 electric wave signals received from a GPS satellite as described above.
The GPS receiver shown in the same drawing is provided with an antenna 1 which receives electric waves transmitted from satellites, an RF (radio frequency) amplifier 2 for amplifying the L1, an RF amplifier 3 for amplifying the L2 constituents, IF (intermediate frequency amplification) circuits 4,5 which are respectively connected to the amplifiers 2, 3 and includes a frequency conversion, A/D conversion circuits 6,7 for digitally converting the IF (intermediate frequency) signals outputted from the IF circuits 4,5, a PLL synthesizer 8 for sending out clock signal to each of the above mentioned circuits, and a TCXO 9 for giving the synthesizer 8 signals issued from a station.
Output signals L1 IF signals, L2 IF signals and clock signals sent from the A/D converting circuits 6,7 are inputted into a DSP (digital signal processor), wherein inverse spread spectrum (hereinafter called SS) modulation signal and data demodulation are carried out.
Furthermore, a digital signal processor (hereinafter merely called DSP), which carries out demodulation operations of a multi-channel type receiver, of the GPS receivers, has four to twelve independent channels. Differing from a sequencing receiver, satellites are independently assigned to a plurality of channels, wherein since transfer waves of the respective channels and the respective tracking circuits of PN code are always locked at satellites assigned to the respective channels, a continuous flow of quasi distance and transfer wave phase measurement values is generated.
FIG.3 shows the interior structure of one channel in a DSP of conventional multi-channel type GPS receiver. In the same drawing, a carrier tracking circuit, a code tracking circuit and a C/A code generator are provided at the L1-C/A code receiving channel side.
Furthermore, a carrier tracking circuit, a code tracking circuit and a P code generator are provided at the L1-P code receiving channel side.
Also, a carrier tracking circuit, a code tracking circuit and a P code generator are provided at the L2-P code receiving channel side.
In the DSP, carrier tracking and code tracking are executed by using each generator. The result is transmitted to the CPU. In the CPU which receives output signals from the DSP, position measurement calculation is carried out on the basis of the data obtained from each channel.
However, when this kind of GPS receiver begins to receive, a state where it is not known which satellite is assigned to the respective channels of the DSP is called Cold Start.
In the cold start, in the case of a receiver having eight satellite channels with respect to twenty-four GPS satellites available at present, any of L1 receiver or L1+L2 receiver is able to search for satellites which are able to receive signals with satellites, assigned to eight L1-C/A code channels one after another.
This is because the tracking element (almanac data) of satellites is old, the initial position is not reliable, or the time is not accurate, or the L1-C/A code cannot be demodulated by the receiving circuit at the L1, L2-P code receiving channel side.
Therefore, even in a case of this kind of multi-channel type GPS receiver, since the satellites to be assigned are not clear in the cold start, such a method has been employed, wherein satellites are assigned to the L1-C/A code receiving circuit one after another and are searched for with respect to which satellites of the twenty-four satellites can be received.
In such conventional GPS receivers described above, more search time is required in the cold start than in the warm start. For the time being, a user must wait. Therefore, shortening of the time required for the cold start was a large technical theme.
The present invention was developed in view of such a shortcoming in the conventional arts, and it is therefore an object of the invention to provide a GPS receiver by which the search time during the cold start can be shortened.
In order to achieve the above mentioned objective, in a multi-channel type GPS receiver which is able to respectively receive a plurality of channels and two kinds of C/A and P codes, a C/A code generator is changeably installed in a P code receiving channel of the receiver, the C/A code generator is operated when starting a cold start, and the P code receiving channel acts as a C/A code receiving channel.
According to the construction, since searching is carried out by using all the channels including the P code channel, it is possible to shorten the search time in comparison with the before mentioned conventional receivers, whereby the power consumption can be decreased.
Furthermore, in a multi-channel type GPS receiver which is able to receive two kinds of C/A and P codes in L1 and L2 frequency bands and is provided with a plurality of channels, a C/A code generator is changeably installed in each P code receiving channel for L1 and L2 frequency bands of said receiver, a C/A code and P code receiving channel for the L1 frequency band and a P code receiving channel for the L2 frequency band are used as they are, a C/A code and P code receiving channel for the L1 frequency band is used for receiving the C/A code for the L1 frequency band, and a C/A code and P code receiving channel for the L1 frequency band and P code receiving channel for the L2 frequency band are used for receiving the C/A code for the L1 frequency band.
According to this construction, a flexible channel configuration is enabled.
As has been made clear from the above description, a GPS receiver according to the invention can shorten the search time during the cold start. Therefore, the waiting time can also be shortened.
Furthermore, as a derivative effect, in a case where some channels of the DSP is accommodated in one IC package, for example, in a case where the L1-C/A, L1-P, L2-P is equivalent to four channels accommodated in one IC package, if a two-frequency receiver of twelve channels uses three ICs and is constructed as a one-frequency L1-C/A 12-channel receiver, only one IC is enough by setting all L1P and L2P in the L1-C/A receiving mode, whereby the efficiency of IC development can be further improved.